June 8, 2004: The Transit of Venus

At the time of writing (November, 2003), the planet Venus has
appeared in the southwestern sky after sunset. It will gradually
pull away from the Sun until next spring, when it reaches
greatest elongation on March 29th. Then in April and May it
accelerates toward the Sun, and passes through inferior
conjunction on June 8th, 2004.

This in itself is nothing special--Venus does this every 19
months and usually passes well north or south of the Sun as it
does so. But one inferior conjunction in about 40 is exceptional,
with Venus crossing the face of the Sun itself, creating a
transit of Venus.

Although one of the rarest astronomical phenomena, transits of
Venus are very predictable and come in cycles of 243 years.
(e.g., 1761/2004, 1769/2012) They also come in series, each
series being all the transits in a cycle. Currently, when one
transit takes place, another happens eight years later. Each
member of an eight-year pair occurs in the same month, which is
either June or December.

Our century two transits are in 2004 and 2012, both
in June, which favors observers in the Northern Hemisphere. The
last pair was in 1874 and 1882, both in December.

Thus our most recent observing experience of a transit of Venus
is 121 years old. We have no digital images or photoelectric
photometry of a transit of Venus. We do, however, have
observers notes, drawings, and film photographs.

The 2004 transit of Venus lasts lasts slightly over six hours,
and of course can be seen only from those areas where the Sun is
shining. This means that about a quarter of the world can see the
entire transit; another quarter sees only the first part of the
transit, and a third quarter can watch only the last part.

In 2004 the favored longitudes that will see the entire event
span most of the Old World, and thus most of mankind. As is
always the case with June transits, anyone in the Arctic, weather
permitting, can watch the transit from beginning to end. However,
the Sun will set before the transit ends in Australia and
easternmost Asia. On the other hand, the Sun will rise with the
transit already in progress for most of the Americas. The eastern
Pacific and western part of North America fall in the zone where
the entire transit occurs at night.

For the imaginary geocentric observer, the transit begins at
05:14 Universal Time and ends at 11:26. The times of transit
phenomena throughout the portion of the world where they can be
seen will differ by at most 7 minutes from the geocentric times.

Choosing an Observing Location

The A.L.P.O. is not organizing tours or observing sites for the
transit, but advertisements for such tours are starting to appear
in astronomy magazines and on the World Wide Web. Also, several
astronomical clubs have announced their observing plans.

For those living there, or willing to travel so far, the Eastern
Hemisphere (except for western Africa, eastern Asia, and
Australasia) is a desirable destination because the transit will
be visible there from beginning to end. Those dwelling in North
and South America should plan to be in the eastern part of their
continents in order to see at least the egress phase of the
event.

Unlike the case with the narrow track of totality for a solar
eclipse, transit travelers can choose an observing place from an
area of millions of square kilometers. In selecting a site, human
issues such as cost, convenience, and local politics are
obviously important. In terms of nature, an area that has a
relatively high mean daily hours (or percentage) of sunshine in
June is desirable. This criterion favors the southeastern United
States for North America. In the Old World, the Mediterranean
Basin, northern Africa, and the Middle East are similarly
favored. The real question is what the weather will be like on
June 8, 2004. Observers who can relocate in response to
short-term weather forecasts and satellite cloudiness images will
have a better chance for clear skies than one at a fixed
location. Finally, the higher the Sun is in the sky during the
transit, the better the chance of avoiding clouds (which, due to
perspective, tend to bunch near the horizon), and of having
better seeing (i.e., less atmospheric turbulence due to a shorter path length through our atmosphere).

Transit Phenomena

The chief interest in transits of Venus in the 18th and 19th
centuries was timing limb contacts in order to determine the
value of the solar parallax, and thus the Earth-Sun distance.
However, even by the time of the 1874/1882 transit pair other
methods for finding the scale of the Solar System had become
competitive, and by now we know these values to far greater
accuracy than can be had by transit observations.

However, a number of other scientifically significant phenomena
are associated with the transits of Venus.

First, within a few days of the transit, Venus will be nearer the
Sun in our sky than in most other apparitions. The two bodies
will appear within 10 degrees of each other from June 2nd-14th,
and within 5 degrees from June 5th-11th. Under these conditions,
Venus appears as a very narrow sunlit crescent, with its horns
faintly extended, sometimes forming a complete circle. This
phenomenon is caused by scattered light in the planet
atmosphere above its cloudtops, and was the first evidence for an
atmosphere of Venus. Observing the horn extension will be
difficult, requiring that either one observe Venus near the
horizon in bright evening or morning twilight; or, while taking
extreme care to avoid accidentally getting the unfiltered Sun in
one field of view, using tube extensions or sunshades
to observe Venus near the Sun in complete daylight.

When the transit itself takes place, the beginning of the
planet entry onto the Sun disk is called
First Contact, the start of Ingress. Second Contact takes place
when Venus completes Ingress. Third Contact occurs when Venus
starts to leave the Sun disk, the beginning of
Egress, which ends at Fourth Contact, when the planet completely
leaves the Sun, ending the transit. Venus crosses the
Sun southeast limb during Ingress (First Contact will
be at position angle 116, 26 degrees south of celestial east),
and then travels southwestward to the Sun southwest
limb (Fourth Contact will be at position angle 216, 36 degrees
west of celestial south).

The Universal Times on June 8, 2004 that the four contacts are
predicted to occur are listed below for some locations worldwide
("---" means that Venus will be below the place horizon at the time):

The notorious black drop occurs near Second
and Third Contacts. when the limbs of Venus and the Sun gradually
separate (Second Contact) or merge (Third Contact). Thus, the
contact timings made by observers even at the same site can
differ by tens of seconds.

The literature sometimes blames the black-drop effect on
Venus atmosphere. However, the same phenomenon
occurs during transits of airless Mercury. Actually, the black
drop is simply due to solar limb darkening and to the inevitable
blurring of any telescope image due to diffraction
and atmospheric seeing.

The one real phenomenon that is unique to transits of Venus is
the ring of light or aureole that appears
on the limb of Venus. silhouetted against the sky during ingress
and egress. The effect is due to refraction of sunlight in the
planet upper atmosphere. The aureole is far brighter
than the horn extension mentioned earlier, but has never been
photographed; all we know of it comes from written descriptions
and drawings. Thus, obtaining photographs and electronic images
of the aureole should be one of the highest priorities for the
2004 transit.

Some historical observers of transits of Venus have reported
anomalous phenomena, such as deformations of the
planet limb, areas or points of light within the dark
hemisphere of Venus, or a halo of light, much wider than the
aureole, around the planet when fully on the disk of the Sun.
Such phenomena are almost certainly due to contrast effects and
light scattering within our atmosphere, or intrinsic to
one telescope, eyepiece, filter, or eye itself.

Observing the Transit of Venus

Observing a transit of Venus involves looking at the Sun, so
every safety precaution that one would use for a solar eclipse or
sunspot observation should be taken when watching or imaging the
transit. The two safe methods are either:

(1) A full-aperture safe solar filter placed securely over
the front end of one instrument (be it eye, camera,
binoculars, or telescope). Do not look through binoculars or a
telescope wearing filters over your eyes rather than over the
instrument aperture itself. Make sure that there are no scratches
or holes in the filter, which if small will reduce contrast, or
if large will pose a danger. Small defects can be painted over
(e.g., with whiteout but large ones will
require replacing the filter. Also, when placing a filter over
the aperture of your telescope, tilt it a degree or so from being
perpendicular to your optical axis to prevent a
host image caused by the highly reflective
rear of the filter.

(2) Eyepiece projection. This is normally fairly safe, but as
the transit lasts over six hours, either your eyepiece or
secondary mirror, or both, may overheat. At the least, this could
degrade the image, but also might destroy your secondary or your
eyepiece (or hurt your eye if next to an overheated eyepiece).

There has been some debate whether Venus can be seen against the
Sun simply with one eyes--safely filtered of course.
Actually, there are numerous reports from the last transit, in
1882, that many people succeeded in this. The writer has
confirmed this by viewing the sun image reflected in
a mirror with a black spot the same apparent size as Venus during
transit (about 1 arc-minute).

However, you may want to use some additional optical equipment,
particularly if you make a special trip to see the event.

Using a minimum of equipment, you should be able to comfortably
watch the event with binoculars--if both lenses are securely
filtered. Likewise, you should be able to photograph the transit
with a filtered telephoto lens--say 200 mm or greater. Exposures
will be short, so an equatorial mounting is unnecessary.

However, if you want a chance of seeing or recording the black
drop or the aureole you will need a telescope. Here,
our most recent observational experience comes from the transits
of 1874 and 1882! Most of the telescopes used by the
19th-century expeditions were in the aperture range of 3 to 8
inches, with 5- and 6-inch clock-driven refractors the most
popular. Observers also used Newtonian reflectors, particularly
if they lived in the visibility zone and ob served from home or
an existing observatory.

Special Projects

(1) Observing Venus off the Photosphere. The possibility of
observing Venus horn extensions during the days before and after
the transit has already been described. In addition, on the day of
the transit itself, Venus may be seen off the Sun photosphere
(surface visible in white light) both before First Contact and
after Fourth Contact. Actually, if one has access to a
coronograph (a rare form of telescope used to observe the
Sun corona outside eclipses) one might spot the planet hours before
and after the official time of the transit. More
probably, using one of the much-more-common Hydrogen-alpha
filters, Venus might be seen perhaps as much as a half-hour
before First Contact or after Fourth Contact, silhouetted against
a prominence or the Sun's chromosphere.

(2) Visual Timing of Transit Contacts. This was the most
popular form of observation during the historical transits
because contact time differences among the widely separated
observing stations could be deduced to
calculate the value of the solar parallax and thus the distance
of the Earth from the Sun. Today we know this value from other
methods to a very high degree of precision, so that contact
timings no longer serve their original purpose. Nonetheless,
observers may wish to conduct such timings as an educational
exercise, in particular combining their timing with those of
others to see how closely their results compare with predicted
times and with the modern value of the solar parallax. Second
and Third Contacts are the best to time, with Second Contact
considered as the moment during ingress when the
filament between Venus and the
Sun limb breaks, when the two limbs are first clearly
separated. Likewise, Third Contact is considered the moment when
the filament forms again during egress. These moments should be
timed to one second precision in Universal Time (UT), with the
time standard either a shortwave time signal (e.g., WWV) or a GPS
signal. Note that such a project is being coordinated by the
European Southern Observatory (see their website:
http://www.eso.org/outreach/eduoff/vt-2004/).

(3) Recording the Appearance of Venus During the Transit. There
are several means of recording the appearance of Venus during the
transit (or even before or after it--see above), especially for
the critical ingress and egress phases.

Basic Documentation.--What will be particularly useful will be
records of transit phenomena--visibility off the photosphere, the
aureole, the black drop, and any anomalous appearances (see
above). Whatever the form of one observational
record, its scientific value depends on adequate documentation.
What is needed for all forms of observation is:
observer name and postal address (and email address
if applicable); latitude and longitude or observing site (to 0.01
degrees or one arc-minute); description of instrument (eye; or
aperture of telescope or aperture and magnification of
binoculars); description of filter used (or of projection
method); atmospheric transparency (on a 0-5 scale, with 5 best
and 0 worst); and Universal Time of each remark, drawing, or
image accurate to one second if possible.

Visual Observation.--Visual observations can take the form of
remarks (including contact timings) or drawings. Refer to
celestial directions either using the words north, south,
preceding (the direction of drift with one drive turned off), or
following (the opposite of preceding); or
as celestial position angle (measured from 0 degrees for
celestial north counterclockwise 360 degrees through celestial
east, south, and west). In referring to directions, correct for
the reversal that occurs if you used a right-angle viewer for
direct visual observation. Note that with projection the
Sun's disk will normally be reversed, but will be
correct with the use of a right-angle viewer. The orientation of
drawings should be clearly indicated. Besides the basic
documentation above, give the magnification used (as well as the
projected disk diameter if you used the projection method), and
the atmospheric seeing on the 0-10 scale (0 is worst and 10 is
perfect).

Still Photography.--Film or digital still photographs taken with
a suitably-filtered telephoto lens should show Venus while in
transit. The camera can be mounted on an ordinary tripod because
the exposure will be very short. However, to capture any of the
transit phenomena mentioned earlier, a telescope will be
necessary, along with a suitable full-aperture filter. The
duration of the exposure can be judged at the time of the transit
with a digital camera, but for a film camera should be found
previously by experimentation with the Sun, and with the same
optical arrangement. Remember that the delicate aureole will
probably require a longer exposure than for the Sun's photosphere.

Even with a safe filter, the Sun's image will be so
bright that a higher-quality medium-speed film or digital camera
sensitivity setting (e.g., ISO 50-100) can be used. The exposure
may still be so short that a clock drive will not be necessary to
prevent blurring. Still, a clock-driven equatorial mounting will
be a great asset in tracking Venus during the six hour-long
transit, although the planet's motion will make it
necessary to make frequent adjustments.

CCD Imaging.--Recording the transit, particularly ingress and
egress, with a CCD camera will allow photometry of the aureole
and the black drop. This is because taking flat frames and dark
frames allows one to correct the image itself for background
noise and variations of sensitivity among the camera's
photosites. Then the CCD response is close to linear, and one
could, for example, use the mean brightness of the center of the
Sun's disk as a standard. For those traveling to a
site, a possible disadvantage of using a CCD camera is that it
must be connected to a computer. Another disadvantage is that
most CCD cameras take monochrome (grey-scale) images, and the to
color images one must take three images in succession through
different color filters.

Video.--The disadvantage of taking still photographs or images is
that there always is an interval between successive images, which
may be a minute or more when making colored CCD images. Thus the
observer may miss rapidly changing events during ingress or
egress. Video allows continuous coverage at a typical frame rate
of 30 frames per second. Admittedly, analog-video frames are
noisy, and several must be stacked to obtain an acceptable image.
Results are definitely better if one uses a digital video (dv)
camera, or records onto a digital-format recorder from an analog
camera.

Webcams.--Webcams provide a stream of digital images, thus also
supplying continuous coverage. Excellent images of the May,
2003, transit of Mercury were obtained with webcams, so this
medium has great potential for the 2004 transit of Venus. As
with CCD cameras, Webcams need to be attached to computers.

Stacking Images.--This process uses a computer to register and
combine anywhere from a few to thousands of individual digital
images, with either manual or automatic selection of the best
images to use. The stacking process can be carried out anytime
after the images themselves are acquired. It is possible to
stack digital still-camera or CCD images, but stacking is most
effective when using the many images provided by video cameras or
webcams. The final result is typically far better in resolution
and contrast than even the best individual frames. However, the
gain in spatial resolution is had at the expense of time
resolution because one necessarily stacks images taken over a
range of time. As Venus will be moving in relation to the Sun by
about one arc-second every 20 seconds, it would be wise to stack
no more than a few seconds worth of images at a time.

General Comments on Photography and Imaging.--Small-scale views
that show Venus' position in relation to the
Sun's limb or to sunspots or other solar features will
provide an interesting and visually striking record of the
transit. Multiple exposures, perhaps combined with computer
image processing, will give a record of the planet's
track across the face of the Sun.

Larger-scale views of Venus in relation to the Sun's
limb, taken simultaneous at widely-separated stations, can be
combined to give a three-dimensional view of the transit.

For transit photographs or images to be of scientific value in
recording such phenomena as the aureole and the black drop, a
large image scale is necessary, even to the extent that Venus
covers a significant portion of the frame. With moderate-size
telescopes this will require either afocal imaging at high
magnification or imaging directly on the film or chip with
eyepiece projection or Barlow-lens extension of the effective
focal length.

Computer processing is possible with all forms of imaging.
Photographs can be scanned and converted to digital images and
analog videos converted to digital with an analog-to-video
frame grabber. Digital video, digital still-camera, CCD,
and webcam images are digital to start with.
Common enhancement techniques include contrast stretching and
sharpening by such methods as unsharp masking. Computer
enhancement should be done cautiously because it can create
artifacts, such as a false light ring
around the planet, or a bright spot on its dark hemisphere.
Certainly, the observer should always retain copies of all
digital images in their unenhanced (raw form, and should document
all enhanced images with comments on the types of enhancements used.

Besides the basic and specialized documentation already
described, all still photographs and images, as well as video or
webcam images, should be documented with the Universal Time of
exposure, exposure time (shutter setting), and effective focal
length of the optical system. Naturally, it is important to note
if the photographs or images are reversed.

Submitting Observations

If you observe the 2004 Venus transit, your efforts will have
permanent value only if they are communicated. All forms of
observation--written notes, drawings, photographs, electronic
images--along with the necessary documentation, should be sent
to: